The thin, wispy clouds known as "cirrus" cover nearly a third of the globe and are found high in the atmosphere – 5 to 10 miles above the surface. But a new study shows that they typically have a very down-to-earth core, consisting of dust and metallic particles.

Cirrus clouds are important to global climate because they interact with radiation from the sun and from the earth. Cirrus are made of ice crystals, but a small "seed particle" is often needed to start the process of forming the ice. Scientists haven't known exactly what kind of particles lead to cirrus formation, causing the climate-related properties of the cirrus to be highly uncertain.

"Cirrus clouds are picky about the kind of particle they form on. Mineral dust and metallic particles are the most preferred types," said Karl Froyd, one of the study's lead scientists and a researcher with CIRES working at CSD. "We've seen hints of this before, but this study demonstrates that it's true across a range of geographic locations and a variety of cirrus cloud types."

Other particle types, including mixtures of sulfate and organic carbon, biomass burning particles, elemental carbon, and particles of biological origin were much less efficient as seeds for cirrus clouds.

Current global models prescribe a variety of cirrus formation mechanisms, leading to a wide range of cirrus properties and associated climate effects. This study simplifies the playing field, suggesting that most cirrus form on a special subset of aerosol particles, and that most of those particles are mineral dust and metallic particles.

"These results are going to allow us to better understand the climatic implications of these clouds in the future," said Daniel Cziczo, a scientist at the Massachusetts Institute of Technology and the lead author of the study.

Photo: K. Froyd, NOAA / CIRES

NOAA's "PALMS" instrument aboard the NASA WB-57 high-altitude research aircraft. The Particle Analysis by Laser Mass Spectrometry instrument characterized the chemical composition of seeds in individual cirrus crystals sampled during several research flights over North America and Central America.

The scientists used instrumentation aboard high-altitude research aircraft to sample the cirrus clouds, in flights over Central America and North America from 2002 to 2011. A special inlet was designed by Froyd to selectively capture ice crystals when flying through cirrus. After removing the water from the cirrus crystal, Froyd and CSD scientist Daniel Murphy used an instrument known as PALMS (Particle Analysis by Laser Mass Spectrometry) to analyze the residual seed core. PALMS is a state-of-the-art instrument, originally developed at NOAA by Murphy, that can characterize the chemical composition of particles one by one as they are sampled into the instrument.

"PALMS gives us a detailed chemical fingerprint of each individual particle," said Murphy.

Also as a part of the study, the scientists used a model to calculate the atmospheric seed particle concentrations using estimates of the emissions of atmospheric fine particles from the Earth's surface, their atmospheric transport, and their ice-forming ability.

"The model results were completely independent of the measurements, yet came to the same conclusions. This gives us confidence that we're not missing something important," said Froyd.

The new study provides many answers about how cirrus clouds form, but also raises new questions. Is it possible that human activities could affect the amount of dust and metallic particles in the atmosphere, and therefore affect the formation of cirrus clouds? Mineral dust comes from arid regions such as the Sahara and Gobi deserts, and atmospheric winds carry them across the globe. Agriculture, transportation, and industrial processes also release dust into the atmosphere. Some studies suggest that desertification, land use change, and changing rainfall patterns could account for 10 to 50% of the dust currently in the atmosphere. Other studies indicate that dust concentrations could increase during this century, perhaps even doubling or more. The metallic particles found by the researchers are from industrial and combustion sources that have decidedly human influences.

The researchers are now focused on understanding exactly how much mineral dust is in the atmosphere, and where the metallic particles come from. And, they wonder if their results would hold true in the southern hemisphere. Most of the large sources of dust and industrial particles are in the northern hemisphere.

"Cirrus may form differently in the southern hemisphere," said Murphy. "That's one of the next frontiers for this research."

Formation of cirrus clouds depends upon the availability of ice nuclei to begin condensation of atmospheric water vapor. While it is known that only a small fraction of atmospheric aerosols are efficient ice nuclei, the critical ingredients that make those aerosols so effective has not been established. We have determined in situ the composition of the residual particles within cirrus crystals after the ice was sublimated. Our results demonstrate that mineral dust and metallic particles are the dominant source of residual particles, while sulfate/organic particles are underrepresented and elemental carbon and biological material are essentially absent. Further, composition analysis combined with relative humidity measurements suggest heterogeneous freezing was the dominant formation mechanism of these clouds.